3rd generation cellular communication systems, such as the Universal Mobile Telecommunication System (UMTS), have been specified to provide a large number of different services including efficient packet data services. For example, downlink packet data services are supported within the 3rd Generation Partnership Project (3GPP) release 5 Technical Specifications in the form of the High Speed Downlink Packet Access (HSDPA) service.
In cellular communication systems, the air interface resource is typically the limiting factor for the achievable capacity of the system and therefore it is of the utmost importance that this is used efficiently. It is therefore important that the scheduling of data packets results in an efficient utilisation of the available resource and significant research and development has been undertaken in order to find reliable, practical and efficient scheduling algorithms.
A Radio Network Controller (RNC) is responsible for allocating air interface resources to each base station in the form of one or more scrambling codes. The RNC will also typically allocate a set of spreading codes that can be used by the base station for carrying HSDPA. The base station (Node B) is then responsible for the scheduling of HSDPA data packets over the air interface and specifically for allocating the codes to the individual data packets or mobile stations. For example, the individual base station is responsible for scheduling HS-DSCH (High Speed—Downlink Shared CHannel) transmissions to the mobile stations that are attached to it, for operating a retransmission scheme on the HS-DSCH channels, for controlling the coding and modulation for HS-DSCH transmissions to the mobile stations and for transmitting data packets to the mobile stations.
In order to reduce the resource usage for HSDPA channels, scheduling is thus performed at the base station rather than at the RNC. This allows scheduling to be sufficiently fast to dynamically follow radio condition variations. For example, when more than one mobile station requires resource from the shared HSDPA channel, the base station may schedule data to the mobile stations experiencing favourable radio conditions in preference to the mobile stations experiencing less favourable conditions. Furthermore, the allocated resources and the coding and modulation applied to transmissions to mobile stations may be highly tailored to the current radio conditions experienced by the individual mobile station. Thus, the fast scheduling performed at the base station allows link adaptation and an efficient resource usage. HSDPA seeks to provide packet access techniques with a relatively low resource usage and with low latency. In addition to using base station based fast scheduling, HSDPA also makes use of other techniques in order to reduce the resource required to communicate data and to increase the capacity of the communication system. These techniques include Adaptive Coding and Modulation (AMC) and a retransmission scheme for lost data packets. In the retransmission scheme, which is referred to as ARQ or Hybrid ARQ (HARQ), the mobile station transmits feedback data in the form of uplink acknowledge (ACK) or non-acknowledge (NACK) messages which are used to determine whether individual data packets need to be retransmitted. If no ACK message is received for a given data packet, this data packet is retransmitted to the mobile station. In HARQ, soft combining of the different transmissions of the data packet is used at the mobile station in order to improve reliability and reduce the required transmit power.
A number of different scheduling mechanisms for cellular systems in general, and HSDPA in particular, have been proposed in the prior art. The most advanced and best performing schedulers tend to be schedulers known as Proportional Fair (PF) schedulers, which perform the scheduling taking into account both channel conditions and achieved throughput fairness. Thus, the PF schedulers seek to provide a reasonable distribution of resources between different users while ensuring that the air interface resource is efficiently used. However, such schedulers are primarily designed for scheduling of streaming traffic and best effort data packet services for which the delay budget is relatively large.
Accordingly, they tend to introduce a variable and potentially large delay. However, an increasing number of communication services are aimed at communication services that have a very specific delay requirement. For example, Voice over Internet Protocol (VoIP) services require the delay to be relatively short resulting in a requirement of the scheduling delay being less than typically a few ten's of milliseconds (ms). Conventional schedulers tend not to be optimum for such services and typically result in reduced radio resource utilisation efficiency and/or longer delays.
Hence, an improved scheduling would be advantageous and in particular a scheduling allowing increased flexibility, reduced scheduling delay, efficient resource utilisation and/or improved performance would be advantageous.